1. Field of the Invention
The present invention relates to a thermoplastic elastomer and a gasket material using the same. More particularly, the present invention pertains to a thermoplastic elastomer and a gasket material using the same that are excellent in sealing properties, prevent the generation of gases such as volatile low molecular components even during a long-term application, and in particular are well suited for use as a member for electronic equipment; to a method for injection molding a thermoplastic elastomer material, and more particularly; to a method for injection molding a thermoplastic elastomer material which is excellent in sealing properties rand sliding properties on the surface of a molding, prevents the generation of gases such as volatile low molecular components even during a long-term application, and particularly, is well suited for use as a member for precision equipment. The present invention further relates to an external anti-sticking agent for rubber or thermoplastic elastomers which is used for adding to the surfaces of products, moldings or pellets comprising rubber or a thermoplastic elastomer, and to rubber or a thermoplastic elastomer which is incorporated with said anti-sticking agent and is excellent in anti-sticking properties.
2. Description of the Related Arts
The development of electronic equipment in recent years really amazes the persons concerned. The aforesaid equipment, which is subjected to printed circuit on a substrate by the use of integrated circuits utilizing semiconductors, is directed to compactified lightweight equipment, and is prone to be readily damaged by moisture or dampness in particular. Accordingly, the hermetically sealing performance of a gasket is a factor of importance for the performance and durability of the electronic equipment, which gasket is used for hermetically sealing the joint surface between a cover body and a box type main body having built-in integrated circuits mentioned above.
It is customary for the purpose of hermetical sealing to integrate a cover body and a box type main body having built-in integrated circuits into equipment by interposing a gasket which hermetically seals the joint surface between said both the bodies and tightening them with fixing bolts. There has heretofore been used a high density urethane foam material as a gasket material. The urethane foam material is foamed urethane cut into thin sheet foam, and is used in many cases by sticking an adhesive tape on the urethane sheet foam and punching the sheet foam into a prescribed shape. In addition, there is proposed a method for injection molding an elastomer after a frame body composed of stainless steel or a synthetic resin is inserted into a mold {refer to Japanese Patent Application Laid-Open No. 283698/1996 (Heisei-8)}.
However, most of both the gasket composed of urethane foam material and the gasket composed of an elastomer generate gases that contaminate a magnetic disc, when practical use of the main body of electronic equipment raises the temperature therein to 40 to 50xc2x0 C. The gases thus generated are often accumulated on the disc of a hard disc apparatus and the like, thereby making it impossible to read out on the hard disc apparatus.
As a countermeasure for eliminating the above mentioned disadvantages, there is provided a mechanism of adsorbing the gases thus generated {refer to Japanese Patent Application Laid-Open No. 302178/1994 (Heisei-6)},and there is provided a gas adsorbent in breathing holes that are made in the main body of electronic equipment to cope with the infiltration from the outside, of gases that contaminate magnetic discs {refer to Japanese Patent Application Laid-Open No. 36548/1994 (Heisei-6)}.
Nevertheless even with the use of the above mentioned provisions, it has been impossible to avoid the disadvantages that the use of electronic equipment is always accompanied by the generation of gases.
The gasket which is interposed between a box type main body having built-in integrated circuits and a cover body at the joint surface therebetween is usually in the form of a gasket which is fitted to a cover body and thus fixed to the cover. It is customary, for instance, to make holes in the cover body and fix a gasket material from both sides of the cover body through the holes. In this case, since the gasket material is exposed on the upper side of the cover body, the gasket material exposed thereon is sometimes upturned or curled when the cover body is inserted into an electronic equipment space which is narrowed accompanying the recent compactification of electronic equipment part items, or when the gasket is brought into contact with an external member at the time of handling work. Thus, there is caused the problem that such upturning or curling is responsible for defective sealing properties. As a countermeasure thereagainst, an attempt is made to improve sliding properties on the surface of a molding by blending silicone polymer in a thermoplastic material. However, also in this case there is caused such disadvantage that low molecular components in the silicone polymer volatilize and thus exert adverse influence upon peripheral precision members.
On the one hand, there is known that use is made of inorganic fine powders such as calcium carbonate and talc and organic fine powders such as polytetrafluoroethylene, polystyrene and polyethylene as dusting powders for preventing the agglomeration of pellets in the case of injection molding by the use of a thermoplastic elastomer material in the form of pellet. However, the conventionally used powders of these types contain impurities in its production step or halogen components or low molecular components due to the adsorption of various volatile pollutants. Accordingly, there is a suspicion that the aforesaid components are discharged in working environments after being processed into moldings, thereby exerting adverse influence upon the performance of precision equipment. In particular, there is caused a practically serious problem in the case where the moldings are employed as gasket materials for a hard disc apparatus.
Since conventional dusting powders are incorporated into thermoplastic elastomer materials after the molding step so as to be present almost uniformly, the surfaces of the moldings reveal the tackiness and stickiness inherent in the original thermoplastic elastomer. Thus in the case of a structure in which molded materials are exposed on the front side (for instance, gasket materials for hard disc apparatus in the form of fit type), the problem remains unsolved in that the gasket materials are upturned, curled or pulled.
There has heretofore been taken the countermeasure for preventing objects of anti-stickiness from sticking to one another by applying coating of an anti-sticking agent to the surfaces of the objects of anti-stickiness such as products, moldings or pellets each composed of rubber or thermoplastic elastomers. There have heretofore been used, as an external anti-sticking agent, inorganic substances containing natural products such as calcium carbonate, talc, magnesium carbonate and mica, and silicones such as silicone oil and silicone polymer.
Nevertheless, because of their high hygroscopicity and oil adsorptivity the inorganic substances containing natural products, when being applied to a product or the like which is composed of rubber or a thermoplastic elastomer and which is the object to be imparted with anti-stickiness, adsorb low molecular components such as oils blended in said product, thereby making it impossible to use the product or the like for a long period of time. On the other hand, it is concerned that silicone, which is costly and most possibly volatilizes and causes electrically defective contact, will raise problems in the case of fabricating optical part items or precision part items by the use of rubber or a thermoplastic elastomer which is externally incorporated with silicone.
Under such circumstances, it is a general object of the present invention to provide a thermoplastic elastomer composition which is excellent in sealing properties and prevents the generation of gases such as volatile low molecular components.
It is another object of the present invention to provide a gasket material which can solve the foregoing problems, when used in particular as a member for electronic equipment, especially a gasket material for a hard disc apparatus.
It is still another object of the present invention to prevent the foregoing upturning phenomenon of gasket materials and the like by enhancing the sliding properties of the surfaces of injection molded articles, and also to provide a process for injection molding a thermoplastic elastomer material to be used for producing moldings wherein volatile components which contaminate precision equipment members are minimized.
It is a further object of the present invention to provide an external anti-sticking agent which is minimized in hygroscopicity, volatility and oil adsorptivity, and which is capable of imparting stable anti-sticking performance to the objects of anti-stickiness such as a product composed of rubber or a thermoplastic elastomer.
In view of the foregoing, intensive extensive research and investigation were accumulated by the present inventors in order to achieve the above mentioned objects. As a result, it has been found that the general object can been achieved by using a specific thermoplastic elastomer composition, that another object can been achieved by using specific powders as dusting powders for pellet at the time of injection molding, and that the further object can been achieved by coating the surface of the object of anti-stickiness such as a product, molding, pellet or the like with fine particulates of ultra-high molecular weight polyethylene.
It being so, the present invention has been accomplished on the basis of the above mentioned findings and information.
Specifically, the present invention provides a thermoplastic elastomer composition (I) which comprises 100 parts by weight of (a) a hydrogenated block copolymer obtained by hydrogenating the block copolymer comprising at least one polymer block composed principally of a vinylic aromatic compound and at least one polymer block composed principally of a conjugated diene compound; 50 to 1000 parts by weight of (b) a non-aromatic base softening agent having a kinematic viscosity at 40xc2x0 C. of at least 100 mm2/sec; and 1 to 100 parts by weight of (c1) propylene homopolymer and/or propylene copolymer comprising propylene as a principal component each being obtained through polymerization by using a metallocene catalyst.
The present invention also provides a thermoplastic elastomer composition (II) which comprises 100 parts by weight of (a) a hydrogenated block copolymer obtained by hydrogenating a block copolymer comprising at least one polymer block composed principally of a vinyl aromatic compound and at least one polymer block composed principally of a conjugated diene compound; 50 to 1000 parts by weight of (b) a non-aromatic base softening agent having a kinematic viscosity at 40xc2x0 C. of at least 100 mm2/sec; and 1 to 100 parts by weight of (c2) propylene homopolymer and/or propylene copolymer comprising propylene as a principal component each free from a lubricant.
The present invention further provides a thermoplastic elastomer composition (III) which comprises 100 parts by weight of (a) a hydrogenated block copolymer obtained by hydrogenating a block copolymer comprising at least one polymer block composed principally of a vinyl aromatic and at least one polymer block composed principally of a conjugated diene compound; 50 to 1000 parts by weight of (b) a non-aromatic base softening agent having a kinematic viscosity at 40xc2x0 C. of at least 100 mm2/sec; and 1 to 100 parts by weight of (c3) propylene homopolymer and/or propylene copolymer comprising propylene as a principal component each having been heat treated.
The present invention still further provides a thermoplastic elastomer composition (IV) which comprises 100 parts by weight of (a) a hydrogenated block copolymer obtained by hydrogenating a block copolymer comprising at least one polymer block composed principally of a vinyl aromatic compound and at least one polymer block composed principally of a conjugated diene compound; 50 to 1000 parts by weight of (b) a non-aromatic base softening agent having a kinematic viscosity at 40xc2x0 C. of at least 100 mm2/sec; and 1 to 100 parts by weight of (c4) propylene homopolymer and/or propylene copolymer comprising propylene as a principal component each having an MFR (melt flow rate) at a temperature of 230xc2x0 C. at a load of 2.16 kgf (21.2 N) being at most 20 g/10 minutes as measured in accordance with JIS (Japanese Industrial Standard) K7210.
The present invention furthermore provides a gasket material, particularly a gasket material for hard disc apparatuses which comprises any one of the thermoplastic elastomer compositions (I), (II), (III) and (IV).
Moreover, the present invention provides a process for injection molding a thermoplastic elastomer material in the form of pellet which comprises using fine particulates of ultrahigh molecular weight polyethylene as dusting powders for preventing agglomeration of the pellet at the time of injection molding.
In addition to the foregoing, the present invention provides an external anti-sticking agent which comprises fine particulates of ultrahigh molecular weight polyethylene having an average particle diameter of 500 xcexcm or smaller, and which is used for rubber or a thermoplastic elastomer; and rubber or a thermoplastic elastomer which is imparted with excellent anti-stickiness by being incorporated with the aforesaid anti-sticking agent.